1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system that processes a spine of a booklet made of folded sheets.
2. Description of the Related Art
When about 20 or more sheets are folded at a time, a booklet may be formed having a vicinity of a spine being curved. The folded state of the booklet described above is insufficient, so that the booklet is soon opened even after it is folded. Therefore, the appearance is degraded. The booklet described above cannot lie flat, so that it is difficult to stack a great number of booklets. In order to solve the problem described above, U.S. Pat. No. 6,692,208 describes a sheet processing apparatus for squaring a spine of a booklet as one of deforming processes in which the spine of the booklet is pressed to be deformed into a rectangular shape.
The invention disclosed in U.S. Pat. No. 6,692,208 relates to a sheet processing apparatus having cramping jaws that hold and fix a booklet with a folded state, a stop plate that projects from the cramping jaws to be capable of regulating the length of the booklet, and a pressing roller that performs a press forming to a spine of the booklet. According to the sheet processing apparatus described above, the spine of the booklet is squared, so that the spine of the booklet can be deformed into a rectangular shape. From the viewpoint of view, the poor-looking appearance described above is corrected, and when a great number of booklets are stacked, the posture of the booklets is stabilized.
However, in the invention described in U.S. Pat. No. 6,692,208, the spine, which projects from the cramping jaws, of the booklet is pressed. Therefore, when the deformation amount is great, the squared spine might spread in the thickness direction of the booklet. This gives a less attractive appearance. When a predetermined pressing force is applied to a spine that projects in a predetermined amount, a deformed amount of the spine becomes excessive when the thickness of the booklet is small, while the deformation becomes insufficient due to the insufficient pressing force to the spine when the thickness of the booklet is great.
The present invention is accomplished in view of the above-mentioned problem, and aims to provide a sheet processing apparatus that can prevent an excessive deformation to a spine of a thin booklet and insufficient deformation to a spine of a thick booklet in order to stabilize the shape of the booklet that is subject to a deforming (squaring) process.